1. Field of the Invention
This invention relates to the field of thermally-imaged films and, in particular, to a lubrication system for thermally-imaged films which improves motion uniformity during imaging to prevent distortion.
1. Description of Related Art
With the increased use of computer hardware and software to generate information in visible multidimensional form such as graphs and graphics, as contrasted to mere numerical listings, there has come a concomitant increased use of plotters to fix such information on a tangible media. Plotters capable of handling the output of such computer systems have been developed and are continually being upgraded to ensure that fast and accurate plots are being produced. Problems have arisen, however, with plotters using thermally-imaged film in which a thermal printing head selectively heats a thermally-active imaging layer on a polyester base, the film generally being four mils thick, thirty-six inches wide and two-hundred feet long, and being transported across the printing head at a continuous rate of about 0.25"/second. Due, however, to the heat produced by the printing head, as much as 500 watts while printing a solid black area to obtain high density, the imaging layer often sticks to the head resulting in non-linear advancement in the transport mechanism and thus perceptible banding and mispositioning of the printed image pixels. In addition, the acoustic noise levels generated by the printing process are greatly increased due to a loud chattering sound caused by the jerky motion from such sticking. In order to overcome this slip-stick motion, it has been proposed to coat the imaging layer with a lubrication layer. Such layers have been used previously in thermal dye transfer processes to coat the back side of the dye donor element so that it will not stick to the thermal printing head. Such a lubrication layer is described in U.S. Pat. No. 4,829,050, entitled "Solid Particle Lubricants for Slipping Layer of Dye-Donor Element Used in Thermal Dye Transfer". While this application of the lubrication film may have been suitable for its intended usage, i.e., on the back side of a dye donor element, it has been proven to be inadequate on the top side of a thermally-active imaging layer due to the lack of control of the amount of heat being transfered to the lubrication layer by the thermal printing head. Such a lack of control not only resulted in all the problems stated in the referenced patent but also introduced an optical distortion not only in the thermally softened imaging layer but also in the lubrication layer. Since the referenced patent was concerned only with the slipping and non-sticking at the moment of transfer, it did not address the issue of the quality and integity of the lubrication layer after the moment of passage from contact with the thermal printing head.
Thus, it is a primary object of the present invention to provide an improved lubrication system for thermally-imaged films.
It is another object of the present invention to provide an improved lubrication system for thermally-imaged films which provides for linear advancement in the transport mechanism and proper positioning of written image pixels.
It is a further object of the present invention to provide an improved lubrication system for thermally-imaged films which reduces acoustic noise generated by the printing process.
It is still another object of the present invention to provide an improved lubrication system for thermally-imaged films which does not result in optical distortion in the imaging layer and in the lubrication layer.
It is a still further object of the present invention to provide an improved lubrication system for thermally-imaged films which prevents sticking of the imaging layer of the film to a thermal printing head.